Because of its very good transmission properties for wavelengths as short as 180 nm, quartz glass is frequently used as a material for optical elements, in particular for lenses in optical systems, which are operated with laser light sources, in particular with excimer laser light sources, at wavelengths in the UV range. An example that can be named for this type of a system is a projection exposure apparatus for applications in microlithography. Projection exposure apparatus of this type are used frequently at operating wavelengths of 248 nm or 193 nm. Other optical systems which are likewise operated in this range of wavelengths are laser-machining systems, exposure systems for the manufacture of flat panel displays, specifically TFT (Thin Film Transistor) displays, systems for TFT annealing, inspection systems for the detection of defects.
A microlithography projection exposure apparatus includes—besides a projection objective for projecting an image of a structure of a reticle onto a light-sensitive substrate (the wafer)—further optical subsystems, in particular an illumination system serving to produce a homogeneous illumination of the reticle. All of these optical systems are made preferably of synthetic quartz glass.
A manufacturing method for synthetic silicon dioxide glass of high transmissivity for ultraviolet radiation with wavelengths as low as 157 nm and a small OH content is described in DE 199 42 443 A1 (which corresponds to U.S. Pat. No. 6,376,401). A special procedure which is referred to as soot process, is said to make it possible to reduce the content of hydroxyl (OH) groups into the range below about 70 ppm while simultaneously minimizing the content of chlorine and metallic contaminations. The aim in trying to minimize the content of OH groups is to obtain an improved transmissivity, based on the assumption that these hydroxyl groups cause an absorption in a band of the ultraviolet range around 165 nm which leads to a lowering of the transmissivity of the quartz glass for radiation with a wavelength of 157 nm.
According to JP 4-97922, a high content of OH groups is said to lead to a reduction of the induced absorption of the glass under UV laser radiation.
An adequate transmissivity of the quartz glass material is however only one prerequisite for its suitability to work in highly complex optical systems such as for example illumination systems or projection objectives for microlithography applications. It is known that exposure to laser radiation with a wavelength of, for example, 193 nm can lead to radiation-induced changes of the density of the quartz glass material which are accompanied by changes of the refractive index. These changes of the optical properties can, among other risks, lead to imaging errors which impose a limit on the useful life of the systems and in some cases necessitate an exchange of components and a readjustment.
An effect that has been known for some time is the radiation-induced increase in the density of the quartz glass material which is accompanied by an increase of the refractive index in the irradiated area. This effect is referred to as compaction and is a frequently investigated phenomenon whose existence can be proven most clearly under irradiation with relatively large energy densities, for example exceeding 0.5 mJ/cm2. As a means of avoiding that compaction will occur to a critical extent at the typical energy densities and wavelengths in the operation of lithography systems, it has been proposed to pre-irradiate the quartz glass material under high energy densities or to compress it mechanically to a higher density, so that the compaction is largely completed already before the quartz glass material is put into service and that, as a result, a material is obtained which is relatively stable at the radiation densities at which it is used (see for example (U.S. Pat. No. 6,205,818 B1 and U.S. Pat. No. 6,295,841 B1).
However, at low energy densities in the range of the energy densities used in lithography systems, a countervailing effect manifests itself which is connected to the radiation-induced expansion of the material and causes a lowering of the refractive index. This effect of a radiation-induced decrease in density is referred to as rarefaction. The effect is mentioned in the articles “Radiation effects in hydrogen-impregnated vitreous silica” by J. E. Shelby in J. Appl. Phys. Vol. 50, pp. 370 ff. (1979) or “Behavior of Fused Silica Irradiated by Low Level 193 nm Excimer Laser for Tens of Billions of Pulses” by C. K. Van Peski, Z. Bor, T. Embree and R. Morton, Proc. SPIE, Vol. 4347, pp. 177 to 186 (2001).
A further aging effect observed in lithography systems, particularly if the lenses are irradiated with polarized light, is the so-called polarization-induced birefringence (PIB). It has however been found that dry synthetic quartz glass materials, meaning materials of low OH content, have particularly low compaction—and PIB values.
As is the case with all synthetic quartz glass materials for excimer laser applications in the UV range of wavelengths, a certain minimum content of H2 is necessary in order to provide the amounts consumed by the laser-induced effects. If no H2 or not enough H2 is introduced into the material during its manufacture, the induced absorption and compaction increase strongly as soon as there is no longer any free H2 present in the glass after it has been exposed to a radiation for a certain length of time. The required minimum content can be calculated from the pulse count and energy density expected during operation for each lens at its particular position in the optical system. In the simplest case, the H2 consumption has a quadratic dependency on the energy density, a linear dependency on the pulse count, and a linear or sub-linear dependency on the reciprocal of the pulse count. A model can be established by measurements of the H2 consumption after exposure to radiation with different energy densities.
In quartz glass materials of low OH content, the hydrogen at higher temperatures bonds with the glass matrix, which leads to isolated Si—H terminations instead of the endless Si—O—Si chain bonds. This effect changes the quartz glass in such a way that the optical performance can no longer be assured. Furthermore, the hydrogen contained in the quartz glass moves out. This shortens the useful life of optical components that are made according to such a method and are used in lithography optics under irradiation with light in the range of wavelengths of about 150 nm to 250 nm, for example 193 nm.
In an attempt to work around this effect, one follows a method where in a first step, a blank of essentially hydrogen-free raw glass is produced and the blank is charged with H2 only after all heat treatments such as sintering, shaping and stress-release-tempering have been completed. A substantially hydrogen-free raw glass in this context means a raw glass with a hydrogen content that is lower by a factor of 10 than the hydrogen content specified for a later application. The charging occurs under normal pressure or a slight overpressure of a few bar in an inert gas atmosphere with an H2 content of 5% or more. The temperature in this process, as a rule, is kept under 600°.
The blanks are usually cast or press-formed into a cylindrical mold. Typical blanks for the manufacture of lenses for a microlithography projection exposure apparatus have a thickness of 20 to 90 mm and, prior to being charged with hydrogen, an H2 content of fewer than 2·1015 molecules/cm3. The charging with hydrogen in order to increase the H2 content by a factor of 10 requires typically a few weeks to months wherein, based on the laws of diffusion, the charge time increases with the square of the thickness of the blank.
To produce a lens for an optical system of the kind described above, in particular for a projection objective or an illumination system of a projection exposure apparatus for lithography applications, a precursor product is cut out of the cylindrical blank, is brought into the desired lens shape through material-removing work procedures, and this precursor product is then made into a lens through finishing processes.
Now, in view of what has been said above, the invention has the objective to provide a method of manufacturing a lens to be used in an optical system for UV light, wherein the process time required for the charging with hydrogen is shortened.